“Chasing Cars” is a song that’s been used during several pivotal moments over the past 14 seasons Grey’s Anatomy — but its usage in Thursday night’s episode may be its most poignant yet.

Warning: This article contains major spoilers from Thursday’s episode of Grey’s Anatomy. Read at your own risk.

At the end of the series’ first Dia de los Muertos-themed offering, a patient educates Meredith (Ellen Pompeo) about the traditions of the Nov. 1 holiday, which celebrators practice in belief that they can be visited by their deceased loved ones. As Meredith is leaving the hospital, viewers see her former colleague and friend George (T. R. Knight) run around a corner; her mother Ellis (Kate Burton) smiling at her from behind; her dog Doc walking ahead of her down a hallway; her sister Lexie (Chyler Leigh) behind a counter; and her husband Derek (Patrick Dempsey) and his best friend Mark (Eric Dane) watching her as she walks out the door — and it was all set to a Spanish-language version of Snow Patrol’s “Chasing Cars” (performed by MOON&SUN feat. Israel De Corcho) that was commissioned by the Grey’s team specifically for this episode.

The scene was months in the making, and now Grey‘s showrunner Krista Vernoff reveals to EW where the idea came from, what went into making it happen, and why.

ENTERTAINMENT WEEKLY: What made you decide to do a Dia de los Muertos episode?
We had our air dates before we had our season planned and Kiley Donovan, who wrote [tonight’s] episode, came into the room and said, “Hey, we have an episode that’s airing on Day of the Dead, can we do a Day of the Dead episode?” When you’re brainstorming a season there are things that come with total clarity and my answer was, “Only if all of Meredith’s dead people can visit her.” Okay, not “only if” — I was thrilled by Kiley’s idea to do a Day of the Dead episode.

Why were you so thrilled?
We’d done Halloween episodes before but this was something special and it was something that we could really do. We were representing a holiday that had never been represented on our show and a community that is under attack in our country right now and that felt important. Representation matters so much. And I had just watched the movie Coco with my daughter, whose name is Coco, and I was so moved to discover the idea that there is a day when our people can come back to us. So Kylie pitched it and I said, “Let’s have [Meredith’s] dead people come visit.”

T. R. Knight, Patrick Dempsey, Chyler Leigh

Bob D'Amico/ABC (3)

What went into making those moments happen?
We talked about it for four months with the visual effects people. You have to really plan a sequence like that. What I love about that sequence is that we have combined some old footage that aired, some old footage that didn’t air — we pulled old dailies — and we shot some new stuff. Doc, we shot. Ellis, we shot. And the idea of that, for me, is that we don’t know if this is in Meredith’s imagination or memory, or if it’s actually happening. For whatever anybody believes, there is a little bit of something.